I recently had trouble with my Warship loco. Despite cleaning wheels, track and removing fibers..it was still not running very well. Using a TINY camel hair brush dipped in the fluid..touch to a tissue to remove excess..I very sparingly applied this at the point where the axles tips contact the retaining plates (red arrows)..a critical point for electrical contact and one not cleaned unless you remove the wheel sets. After a few minutes of running in my Class 42 is running as well as it was when brand new..and WAY better than before. if you do this go very very sparingly and get the tiniest camel hair brush that you can.

Your findings are very similar to my own.... When I was preparing for Model Rail Scotland, I did some very close and careful examination of the two power units on the class 170 to try and establish why they were running so badly over the two days of the Newbury show. I decided to do a deep clean of the wheelsets....and I found huge (in T terms) lumps of crap jammed in the dimples on the faces of the wheels and around the pivot plates.....exactly where Graham has indicated in his post.

Here's a very badly taken photo of the kitchen towel I used to do the cleaning on:

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A wheelset can be made out with some fluff around the axle....but the important bits are all of the other black bits. All were levered out of the machined dimples in the outer faces of the wheels. This is one DMUs worth!!!

I followed this exercise up with doing the same deep clean on my two HST sets and three or four locos. Most had smaller lumps of crap!!

On day one of Model Rail Scotland, all power units ran without any power-loss related stutters. I repeated the cleaning exercise in between the days, which seemed a waste of time as there was little build-up to remove.....but a well worth exercise! I've never had so much good running before!

So...bottom line....yes, these are crucial electrical contacts....but I would suggest removing the axles and cleaning, then reassembling dry!

You're close, Graham.....but dirt is not collecting on the face. It collects in the machined indent in the centre of the face.

Here's a photo of one of the DMU power unit wheelsets that I had running at the Nottingham show:

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The face is quite clean...but in the centre you can't see the shine on the surface of the indent...because its covered in a black layer of non-conductive debris! This is the electrical contact point between the wheel and the brass plate inside the side of the bogie that transfers power to the bogie retaining spring.

As I said above....cleaning these indents and the brass plates and re-assembling dry seems to be the solution to bad running.